Re: [PATCH v2 10/14] objtool: Add support for relative switch tables

[Christophe Leroy]

Le 22/06/2023 à 13:48, Peter Zijlstra a écrit :
> On Thu, Jun 22, 2023 at 12:54:32PM +0200, Christophe Leroy wrote:
>> On powerpc, switch tables are relative, than means the address of the
>> table is added to the value of the entry in order to get the pointed
>> address: (r10 is the table address, r4 the index in the table)
>>
>>        lis     r10,0		<== Load r10 with upper part of .rodata address
>>            R_PPC_ADDR16_HA     .rodata
>>        addi    r10,r10,0		<== Add lower part of .rodata address
>>            R_PPC_ADDR16_LO     .rodata
>>        lwzx    r8,r10,r4		<== Read table entry at r10 + r4 into r8
>>        add     r10,r8,r10	<== Add table address to read value
>>        mtctr   r10		<== Save calculated address in CTR
>>        bctr			<== Branch to address in CTR
>>
>> But for c_jump_tables it is not the case, they contain the
>> pointed address directly:
>>
>>        lis     r28,0		<== Load r28 with upper .rodata..c_jump_table
>>            R_PPC_ADDR16_HA   .rodata..c_jump_table
>>        addi    r28,r28,0		<== Add lower part of .rodata..c_jump_table
>>            R_PPC_ADDR16_LO   .rodata..c_jump_table
>>        lwzx    r10,r28,r10	<== Read table entry at r10 + r28 into r10
>>        mtctr   r10		<== Save read value in CTR
>>        bctr			<== Branch to address in CTR
>>
>> Add support to objtool for relative tables, with a flag in order to
>> tell table by table if that table uses relative or absolute addressing.
>>
>> And use correct size for 'long' instead of hard coding a size of '8'.
> 
> Again, see tip/objtool/core...
> 
> This one is going to be a little hard. It would be very good if you can
> avoid growing struct reloc; Josh went through a ton of effort to shrink
> it.
> 
> Would it work to use reloc->sym->is_rel_jumptable ? That still has a few
> spare bits in it's bitfield.
> 

I noticed that the relocation types are different, so the flag is indeed 
not needed. I have now done the following, will it work for x86 ?

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index ae4a1608d97e..05d789e6d3b6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1988,7 +1988,7 @@ static int add_jump_table(struct objtool_file 
*file, struct instruction *insn,
  	struct symbol *pfunc = insn_func(insn)->pfunc;
  	struct reloc *table = insn_jump_table(insn);
  	struct instruction *dest_insn;
-	unsigned int prev_offset = 0;
+	unsigned int offset, prev_offset = 0;
  	struct reloc *reloc = table;
  	struct alternative *alt;

@@ -2003,7 +2003,7 @@ static int add_jump_table(struct objtool_file 
*file, struct instruction *insn,
  			break;

  		/* Make sure the table entries are consecutive: */
-		if (prev_offset && reloc_offset(reloc) != prev_offset + 8)
+		if (prev_offset && reloc_offset(reloc) != prev_offset + 
elf_addr_size(file->elf))
  			break;

  		/* Detect function pointers from contiguous objects: */
@@ -2011,7 +2011,12 @@ static int add_jump_table(struct objtool_file 
*file, struct instruction *insn,
  		    reloc_addend(reloc) == pfunc->offset)
  			break;

-		dest_insn = find_insn(file, reloc->sym->sec, reloc_addend(reloc));
+		if (reloc_type(reloc) == R_ABS32 || reloc_type(reloc) == R_ABS64)
+			offset = reloc_addend(reloc);
+		else
+			offset = reloc_addend(reloc) + reloc_offset(table) - 
reloc_offset(reloc);
+
+		dest_insn = find_insn(file, reloc->sym->sec, offset);
  		if (!dest_insn)
  			break;


Christophe